1) Field of the Invention
The present invention relates to a base station apparatus for a radiocommunication network, a method of controlling communication across a radiocommunication network, a radiocommunication network system, and a radio terminal apparatus, all of which are suitable for use in a radio LAN (Local Area Network).
2) Description of the Related Art
FIG. 12 is a block diagram illustrating a communications system to which a radio LAN is applied. In the communications system illustrated in FIG. 12, two radio base stations 10A and 101B connected to a cable network 104 constitute radio LANs (radiocommunication networks) 100A and 100B, respectively.
These radio LANs 100A and 100B are systems, each of which connects a plurality of radio terminals (terminal stations) 102 to the network without a cable. These radio base stations 101A and 101B control a plurality of radio terminals 102 existing in each of radio areas 103A, 103B by periodically sending synchronous frame signals called beacon signals. In short, the range to which beacon signals propagate can be defined as each of the radio areas 103A, 103B of the radio LANs 100A, 100B.
Accordingly, the radio terminal 102 in either the radio area 103A or 103B is capable of establishing communication with a cable terminal 105 connected to the cable network 104 or with another radio terminal 102 existing in either the radio area 103B or 103A, via the radio base station 101A or 101B.
A spread spectrum (hereinafter abbreviated as SS) scheme Is employed for a radiocommunication scheme used for the previously-described medium-speed radio LAN. In contrast to a scheme which employs specifically-limited ordinary frequency bands, the SS scheme utilizes a much wider signal band. In the SS scheme, when a certain frequency is considered, communication is established at an output as low as the level of noise.
As illustrated in FIG. 13, according to the SS scheme, a train of input pulses is subjected to a narrow bandwidth modulation (i.e., primary modulation), and the thus-modulated signal is subjected to spread modulation (i.e., secondary modulation), so as to intentionally spread the spectrum of the signal. The signal subjected to spread modulation is then transmitted. Compared with the original narrow-band modulated signal, the signal having a spread spectrum has a high degree of redundancy and has high resistance to noise or fading. On a receiving side, a received signal is subjected to secondary demodulation (spread demodulation), and the signal is further subjected to primary demodulation, whereby a train of output pulses is obtained.
The SS scheme is further classified into a direct sequence (hereinafter abbreviated as DS) scheme and a frequency-hopping (hereinafter abbreviated as FH) scheme. The DS scheme is a scheme in which information is subjected to secondary modulation through use of a train of noise-like pulses which is much faster than a train of input pulses encoded from information to be transmitted. The FH scheme is a scheme in which a frequency band having a predetermined width is divided into a plurality of channels, and the plurality of channels are switched one after another so as to be sequentially used in the form of a predetermined pattern (i.e., an FH pattern) as a carrier frequency of an ordinary narrow-band modulated signal. Both the DS and FH schemes are intended to allow many users to effectively use a frequency band by distributing the frequency band of the carrier wave so as to reduce transmission time to as short a period as possible.
The previously-described radio LAN employs the FH scheme. When the radio LANs 100A and 100B previously described with reference to FIG. 12 carry out communication according to the FH scheme, the radio base stations 101A and 101B in the respective radio LANs 100A and 100B notify the radio terminals 102 in the radio areas 103A and 103B of FH patterns used in the radio areas 103A and 103B by means of the foregoing beacon signals.
If an FH scheme network is solely present, the network provides the throughput performance inherent therein without radio interference unless another piece of apparatus which sends radio waves at the same frequency band is present in the vicinity of the network. However, if there is another radio LAN system in the vicinity of the network (for example, 10 there are overlapping areas among a plurality of radio areas 103A to 103C as illustrated In FIG. 14), the same frequency band is used in these radio areas. Consequently, there may be cases where the same frequency is used at the same time or where adjacent frequencies are used, the throughput of the networks is deteriorated by radio interference between the networks. As the number of peripheral networks increases, the degree of interference increases, which in turn results in an increase in the rate of deterioration of the throughput. In FIG. 14, 100C designates a radio LAN, and 101C designates a radio base station used in the radio LAN 100C. 103C designates a radio area of the radio base station 101C.
In Japan there are 23 channels in a frequency band which can be used for the foregoing LAN system. With the FH scheme, an operation is repeated in such a way that the 23 channels are sequentially switched one after another in accordance with a predetermined FH pattern. Accordingly, during the course of one round of hopping among 23 channels in accordance with the predetermined FH pattern, if there is present a radio wave having a frequency that interferes with the frequency band, interference occurs once at a frequency that coincides with the frequency of the interference radio wave and occurs twice at frequencies adjacent to the frequency of the interference radio wave.
If interference occurs at a frequency that coincides with the frequency of the interference radio wave (i.e., when the frequency used for the FH scheme matches with the frequency of the interference radio wave), communication can be established by evenly using that frequency band. However, in the case where interference occurs at the adjacent frequencies (i.e., when the frequency used for the FH scheme is adjacent to the frequency of the interference radio wave), such interference cannot be avoided. If there is another radio station which may cause radio interference in the vicinity of a radio station, the communication performance of the radio station decreases by a maximum of 2.5/23=10.8%. For example, if there are five radio stations which may cause radio interference in the vicinity of the radio station, there will be a maximum reduction of 12.5/23=54% in the communication performance of the radio station.
To prevent such a problem, if there are a plurality of networks in the radio LAN system that employs the FH scheme, instead of avoiding the radio interference, there is used a hopping pattern which prevents uneven occurrence of frequency interference, assuming that frequency interference occurs at a certain probability. According to the technique disclosed in, e.g., Japanese Patent Application Laid-Open (Kokai) No. 7-15443, if a certain radio station carries out communication through use of a predetermined FH pattern and if there are networks which use the same FH pattern in the vicinity of that radio station, the radio station prevents frequency interference through using another FH pattern instead of the original FH pattern. However, even if a different FH pattern is used, there is a sufficient risk of interference being caused when the frequency used for the FH scheme matches with the frequency used in the other network or when the frequency used for the FH scheme is located adjacent to the frequency used in the other network. Therefore, it is impossible to ensure prevention of radio interference.
In contrast, the transmission-line performance of the radio LAN is usually about 1-2 Mbps (about 1/10 to 1/5 that of existing cable LANs). Accordingly, if a plurality of radio terminals concentrate at one radio station, there arises the imbalance of load between the base stations having a large number of radio terminals connected thereto and base stations having a smaller number of radio terminals connected thereto. There is a significant difference in performance between the radio terminals used in the same area.
CSMA/CA (Carrier Sense Multiple Access/Collision Avoid)-with-acknowledgment scheme of a MAC (Media Access Control) layer protocol used for radio LAN is analogous to a CSMA/CD (Carrier Sense Multiple Access/Collision Detect) scheme which has come into widespread use for existing cable LANs. However, they are different from each other in the following two points.
First, in the radio LAN, a control frame (a synchronization frame signal) is sent from the radio station at constant intervals, and each of the radio terminals also sends the control frame at irregular intervals, thereby hindering the transmission of an ordinary data frame.
Second, in the radio LAN, collision cannot be detected definitely. Therefore, the following procedure is performed in order to confirm transmission of a frame. The receiving side sends an acknowledgment signal back to the sending side, and the sending side receives the acknowledgment signal in order to confirm that a frame has been correctly received. Therefore, the transmitting side cannot acknowledge the completion of transmission of a frame without collision until it receives an acknowledgment from the receiving side.
In order to determine at an early time collision between (or nonarrival of) data, or in order to ensure the period of time during which a transmission path is occupied, RTS (Request-to-send) and CTS (Clear-to-send) control frames are exchanged between the transmitting side and the receiving side before data transmission. However, if the RTS and CTS frame control signals are exchanged in a state where there is a slight load on a transmission path and there are a small number of collisions, sending one frame of data takes a longer period of time.
Such a complicated protocol is apt to increase the difference in effective throughput between the radio LANs and the cable LANs. Therefore, in some network applications, when the radio LAN is used with these applications, users feel that the response is noticeably bad.